The present invention relates to tunable filters.
Tunable filters, such as Fiber Fabry-Perot tunable filters (FFP-TFs), can be used in connection with laser sources e.g. for suppressing source spontaneous emission (SSE) noise representing an undesired optical power in parts of the spectrum other than the lasing frequency itself. However, the central wavelength in transmission spectrum of the tunable filter must well match with the wavelength of the laser signal in order not to unduly decrease optical power of the laser signal. The filter is therefore tuned in wavelength to ensure that the maximum of the tunable filter pass band is fixed to the frequency of the peak lasing intensity.
Beside an initial adjustment of the filter maximum to the laser peak frequency, a continues adjustment might be required in order to correct for mismatches between filter and laser caused e.g. by tuning the laser frequency, thermal variations in the filter, or frequency drift in the laser.
In U.S. Pat. No. 5,552,919, the central wavelength in the transmission spectrum of the tunable filter is periodically and minutely modulated. An error signal is generated for use in tracking the central wavelength of the filter to the wavelength of the laser by analyzing the modulated filter output.
Although no details of an “Automatic Laser Tracking Filter—ALTF”, as published by Micron Optics, Inc. in its “2001 Test Instruments” brochure, see http://www.micronoptics.com/altf.htm, are disclosed, it appears that the ALTF makes use of the same principle as disclosed in the aforementioned U.S. Pat. No. 5,552,919, i.e. dithering the filter characteristics for deriving a control signal to displace the filter characteristics. After passing the tunable filter, a small fraction of the optical power is tapped off and sent to a detector. The electrical signal from the detector is fed to a scan and lock circuitry. A phase lock loop (PLL) ensures that the maximum of the tunable filter pass band is fixed to the frequency of peak lasing intensity.